1. Field of the Invention
The present invention relates to a novel mount for a lamp which ensures an excellent pressure resistance and a longer service life of the lamp, and to a lamp seal structure employing the mount.
2. Description of the Prior Art
In recent years, ultra-high pressure mercury lamps having a high optical efficiency have gained an increasing share for use as light sources for optical systems (projectors, rear projection TV sets, fiber optics and the like). The ultra-high pressure mercury lamps generally have an internal pressure of not lower than 120 atm at illumination thereof for improvement of the optical efficiency. Some of the ultra-high pressure mercury lamps in production have an internal pressure of about 190 atm. With such a trend, it is very important to improve the pressure resistance of lamp envelopes.
Weak portions of a lamp envelope for use in such an ultra-high pressure mercury lamp are a tip tube trace (i.e., a seal-cut portion) which is formed by seal-cutting a tip tube after filling one or some kinds of required gas (e.g., Ar and Xe) and one or some required substances (e.g., Hg and metal halide) in a light emitting tube portion, and seal portions protruded from both sides of the light emitting tube portion in which a metal foil is embedded. The ultra-high pressure mercury lamp employs a well-known method for producing a tipless type lamp which is free from the generally weak seal-cut trace. However, the seal portions cannot be eliminated. Therefore, the pressure resistance of the seal portions which contain the metal foils directly embedded therein and ensure gas-tight sealing is the most important requirement for the production of the ultra-high pressure mercury lamp.
Ultra-high pressure mercury lamps with an internal pressure of about 190 atm have already been in production. The only method for improvement of the pressure resistance of seal portions of such an ultrahigh pressure mercury lamp is to improve the tightness of the contact between a metal foil embedded in the seal portion and the glass constituting the seal portions of the lamp envelope having a greater thickness. This is achieved by heating the thick seal portion and keeping the inner part of the seal portions at a higher temperature for a longer period.
However, the very thin metal foil embedded in the seal portion is subjected to the high temperature for a long period, during the heating thereby it has a reduced mechanical strength, when the seal portion is heated to be softened in this state and then pressed from opposite sides in dies for pinch-sealing thereof or allowed to be shrunk for shrink-sealing thereof, for example, the weakened metal foil is distorted and, in the worst case, broken due to flow of the glass during the sealing, so that the yield is remarkably reduced by the sealing. In the case of the tipless lamp envelope which has no tip tube, one end of the lamp envelope is first sealed and, after one or some kinds of required gas and one or some required substances are filled in the light emitting tube portion, the other end of the lamp envelope is sealed. After the second sealing, a pressure resistance test cannot be performed on the tipless lamp envelope (in the case of the lamp envelope provided with the tip tube, the pressure resistance test is conventionally performed by charging a high pressure gas into the light emitting tube portion through the tip tube before one or some kind of required gas and the required substances are filled in the light emitting tube portion after the sealing). Accordingly, some of seemingly acceptable lamps have a low pressure resistance, so that the product reliability is reduced by them.
Even if the metal foil is kept in intimate contact with the glass of the seal portion in the aforesaid manner, another weak point resides in the seal portion. That is, a multiplicity of minute cracks occur in the seal portion at cooling after the sealing. More specifically, as the seal portion is cooled after the completion of the sealing, a difference in contraction occurs between the glass seal portion and rod portions (e.g., electrode rods and in-lead rods connected to a filament) of a light emitting element, which are embedded in the seal portion and each have a volume, so that the rod portions which have firmly adhered to the glass seal portions are separated from contact surfaces of the seal portions. At this time, the minute cracks are produced in the contact surfaces.
The multiplicity of minute cracks occurring in the seal portion are developed little by little by repetitive turn on and off of the lamp. Consequently, burst of the lamp is started from the cracks thus developed, resulting in breakage of the seal portion. The risk of the burst of the lamp is increased as the internal pressure of the light emitting tube portion is increased. For further size reduction, higher optical efficiency and longer service life of the lamp of this type (metal halide lamp, halogen lamp and the like), it is inevitable to increase the internal pressure of the light emitting tube portion. Therefore, prevention of the minute cracks in the seal portion is critical. That is, the tightness of the contact between the seal portion and the metal foil profoundly influences the pressure resistance, while the minute cracks in the seal portion profoundly influences the product service life.
Moreover, there is a demand for further improvement of light distribution from the lamp mounted in a system, so that highly accurate centering of the lamp with respect to the system is required.
In view of the foregoing, it is an object of the present invention to provide a mount for a lamp and a highly pressure-resistant and longer-life lamp employing the mount, which feature a very high pressure resistance, a longer service life and a higher production yield with no possibility of deformation or breakage of a metal foil at sealing, and ensure easier centering of a light emitting element with respect to a lamp envelope to meet the demand for further improvement of the light distribution.
First means for achieving the aforesaid object is to provide a separable layer on a surface of a portion of a light emitting element such as in-lead rods or electrodes to be embedded in seal portions of a lamp.
With this arrangement, if the in-lead rods or electrodes of the light emitting elements contract to a greater extent than the seal portions of a glass at a cooling stage after sealing and minute gaps occur therebetween, the separable layers provided on the surface of the portions embedded in the seal portions adhere onto inner surface of the seal portions and are easily separated from the embedded portions (or from the inner surface of the seal portions), thereby preventing occurrence of minute cracks on the inner surface of the seal portions. As a result, the pressure resistance of the seal portion can remarkably be enhanced, and the service life of the lamp can remarkably be extended because the pressure resistance of the lamp can drastically be increased.
Second means for achieving the aforesaid object is to embed a metal foil in a glass bead and then embed the metal foil with the glass bead in a seal portion provided at both ends of a lamp envelope for sealing. With this arrangement, a stress generated by flow of the glass melted at the sealing is not directly exerted on the metal foil, so that the deformation and breakage of the metal foil is less liable to occur at the sealing.
Third means for achieving the aforesaid object is to embed and seal a glass rod block with a light emitting element (i.e., in-lead rod with a filament or electrode rod) fitted in a bore formed in one end of the glass rod block and with an outer lead rod fitted in a bore formed in another end of the glass rod block in each seal portion of a lamp envelop. With this arrangement, the light emitting element can easily be centered with respect to the lamp envelope.